A surfactant has been defined as a solute which, when dissolved in a designated liquid, will alter the surface or interfacial characteristics of the solution in some manner, such as lowering the surface tension.
Surfactants have utility, for example, as detergents, emulsifiers, foamers, wetting agents, dispersants, flocculants and penetrants. They are particularly useful in producing detergent foams of high stability and high foaming power in aqueous systems. Other specific uses include foam stabilization and emulsification of water-organic solvent mixtures.
Synergystic interactions between surfactants, defined rigorously below, occur when two surfactants used in concert provide unexpected surface characteristics beyond those which would be predicted based on the sum of the surface properties of the individual components. Synergystic surfactant mixtures not only have the areas of utility as described above for single surfactants, but also have the added advantage of being able to effect these utilities at lower concentrations. This advantage may be exploited in a variety of ways such as the improved economics of using less material to achieve the same performance, or by extending the effective lifetime of a surfactant solution at a given concentration.
U.S. Pat. No. 3,562,786 of Bailey, et al., first suggested using organosilicone-oxyalkylene block copolymers in combination with organic surfactants to improve the properties of the water soluble organic surfactants for use in aqueous systems. This patent does not deal with the carboxyl group-containing siloxane-polyoxyalkylene copolymers which are the subject of this application, and does not indicate that the disclosed compositions exhibit synergism.
U.S. Pat. No. 4,784,799 of Petroff discloses surfactant compositions comprising a silicone sulfobetaine surfactant and an alkylbenzenesulfonate anionic surfactant, and states that these exhibit synergism. The experimental data presented in this patent show surface tensions produced by 0.1% levels of blends of 2 surfactants, and surface tensions produced by various levels of each surfactant separately, the concentration in each case being that of the particular surfactant in the surfactant blend. Synergism is traditionally expressed as the whole being greater than the sum of the parts. Somewhat more rigorously, it can be defined as two or more agents operating together to cause an effect which is greater than that which would have been expected based on the sum of the effects of the agents operating separately. Petroff provides no indication of the surface tensions which would have been expected based on the sum of the effects of the several surfactants operating separately. He merely showed that each surfactant, considered separately at the level employed for the particular material in the surfactant blend, produced a surface tension higher than that of the corresponding blend of surfactants containing the given level of the surfactant under consideration. Thus, this reference does not describe synergystic surface tension lowering behavior, or suggest a method of achieving synergystic interactions between any silicone surfactants and anionic surfactants.
Rosen has published on "Molecular Interaction and Synergism in Binary Mixtures of Surfactants", setting forth theoretical and test protocols for ascertaining when synergism exists in mixtures of surfactants. See M. J. Rosen, Phenomena in Mixed Surfactant Systems, (J. F. Scamehorn, Ed.), ACS Symp. Ser., Vol. 311, p. 144 (1986).
Synergism in mixed micelle formation is defined by Rosen as being present when the critical micelle concentration of any mixture is lower than that of either pure surfactant. To treat this concept mathematically, a mixed micelle parameter, B.sup.m, is derived which expresses the extent of interaction between two given surfactants in a solution. Thus, according to Rosen: ##EQU1## where C.sup.m.sub.12 is the critical micelle concentration for the mixture;
C.sup.m.sub.1 is the critical micelle concentration of surfactant 1; PA1 .alpha. is the weight fraction of surfactant 1 in the initial surfactant solution; and PA1 X.sup.m is the weight fraction of surfactant 1 in the surfactant of the mixed micelle. PA1 C.sup..sigma..sub.1 is the concentration of surfactant 1 required to achieve a specific surface tension; PA1 .alpha. is the weight fraction of surfactant 1 in the initial surfactant solution; and PA1 X.sup..sigma. is the weight fraction of surfactant 1 in the surfactant in the mixed monolayer.
The value of X.sup.m is derived from the iterative solution to the equation ##EQU2## where C.sup.m.sub.2 is the critical micelle concentration of surfactant 2.
With these mathematical relationships established, synergism in mixed micelle formation is defined mathematically as existing when .beta..sup.m is negative, and .vertline.ln(c.sup.m.sub.1 /c.sup.m.sub.2).vertline.&lt;.vertline..beta..sup.m .vertline..
Similarly, synergism in surface tension reduction efficiency is defined by Rosen as being present when a given surface tension can be attained at a total mixed surfactant concentration lower than that required of either surfactant by itself. To treat this concept mathematically, a parameter .beta..sup..sigma. is derived which expresses the extent of the interaction between the two given surfactants in a solution. Thus, according to Rosen: ##EQU3## where C.sup..sigma..sub.12 is the concentration of the mixture required to achieve a specific surface tension;
The value of X.sup..sigma. is derived from the iterative solution to the equation ##EQU4## where C.sup..sigma..sub.2 is the concentration of surfactant 2 required to achieve a specific surface tension.
With these mathematical relationships established, synergism in mixed monolayer formation is defined mathematically as existing when .beta..sup..sigma. is negative, and .vertline.ln(C.sup..sigma..sub.1 /c.sup..sigma..sub.2).vertline.&lt;.vertline..beta..sup..sigma. .vertline..
It is always desirable to have new surfactant compositions which afford unexpectedly good surfactant properties, for the reasons discussed above. Such compositions are the subject of the present application.